Metal cleaning process



Patented Feb. 6, 1940 pany, Wllmin No Drawing;

This invention relates to'a' process for clean ing metal and more particularly'tc a method for removing oxide; scale" from metal surfaces.

I In the removal of oxide scale from iron, steel and other metalsthe commonmethod'is to immerse the metal in a bath of a strong acid solution. For example, steel is commercially descaled by dipping'in solutions of acids such as sulfuric 'acid, hydrochloric acid and the like. Various materials are added to modify the action of the acid to inhibit the action of the acid on steel after theoxide scale has been removed. A,.com-

mon disadvantage of such acid picklin operations is that steel thus treated, after removal from the pickling bath. tends to rust very easily.

Consequently, unless the metal is covered imme-' dlately with a rust-resistant coating, as by electroplating, it is necessary to. cover the cleaned surface with oil, or grease or similar protective coating to prevent rusting while in storage.

' An object of the present invention is to provide a new and improved method for removing oxide coatings from metal surfaces.

be treated to remove oxide scale and simultaneously be provided with a corrosionrresistant surface. Other objects will be apparent :from the following description of my invention. 7

I have discovered that when oxide-coated iron,

steel or similar ferrous metal is treated with a' sodium-lead alloy at a sufllciently elevated temperature, the oxide scale thereon issubstantially completely removed in a very short period of time, leaving a surface which is highly resistant to rust formation. In order that the sodiumleadalloy treatment may effectively remove the oxide coating it is essential that the operation be carried out at a temperature not lower than about 500 (2.; preferably I carry out the operation at a temperature of 600-800 C. v

I am aware that it has previously been proposed to provide steel with a coating of lead by immersing the steel in fused bath of lead-sodium alloy at a temperature of 380 to 420 C. (720 to 820 F.). The present method is distinctly different in that such high temperature is used that the lead coated product of the prior method is not formed.

A convenient method of practicing my lnven- Application September 8, 1958, Serial N0. 229,008 v I A'further object is to provide a method whereby steel may z, 1,'9s0' UNITED ism-res; PATE T- oFFlca i 0 2,188,930

METAL cinema mocess cease Arthur Vincent-Davies, Wilmington, m1. assignor to E. I. du Pont de Nemours & ComgtomDeL, a corporation Delametal concentration. The alkali metal concen tration may be varied considerably but in general' it will not be less than 0.5% by weight.

While in general I prefer to use an alloy containing 240% by weight of sodium the concentration may be varied-over wide limits. It

into .contact with the bath surface or carried into the bath on the surface of the articles being treated. Hence, in order to maintain the bath constantly 'eflective, alkali metal should be added to the bath from time to time, to maintain a suitable concentration of alkali metal in the bath. When the steel is removed from the bani after having been immersed therein for a sufficient length of time, the oxide scale will have'completely disappeared and loosely adhering droplets of the lead alloy are on the steel surface: Usually, it is necessaryonly to keep the steel in the bath to bring it to the temperature of the bath, to accomplish this result. If the o'iiide scale is heavy.,,1t is sometimes necessary to keep the steel in'the bath for severaL minutes after coming to the bath temperature. Longer times may .be employed, without deleterious effect .on

the steel surface. If the time of treatmentselected does not completely remove the oxide,

' the steel may be reimmersed in the bath until the desired effect is obtained or, if desired, the

steel may be subjected to the next step in the process, for removing the droplets of lead alloy. These may readily be removed by a simple mechanical operation .suclr? as wiping, scratchbrushing, scraping or the like. After such mechanical treatment; the steel will have a smooth,

dark surface, which is highly rust resistant. The

corrosion resistant coating is microscopically thin, hard, durable and very adherent. The mechanical treatment, to remove the loosened oxide scale, is essential in order to produce the rust-proof surface on iron or steel. If this step is omitted, little or no'rust-prooflng effect is obtained. On the other hand, when the mechanical treatment is used'as above described, excellent rust-proofing is obtained.

The above mentioned mechanical treatment to remove the drops-of lead alloy should be sufllciently gentle so as not to cause substantial scratching or abrasion of the steel surface, ii the rust-proofing effect is desired. Usually thebest results are obtainedby washing the surface with water before, during, or after the mechanlcal operation for removal of the lead alloy drop- In one modification of my invention I may cover the molten metal bath with a layer of a suitable material which is substantially chemically inert to the bath and to the metal being treated, in order to inhibit or prevent contact of the bath surface with the surrounding at.- mosphere. Materials for this purpose may be gaseous, solid or liquid. For example, I may maintain the bath in an atmosphereof a nonoxidizing gas such as nitrogen, hydrogen or hydrocarbon vapor. The entrance of oxygen or moisture also may be inhibited by maintaining a layer of carbon, sand, or other similar solid material which will float on the surface of the lead alloy. Preferably, I prevent contact of air with the bath by maintaining thereon a floating layer of fused salt, fused alkali metal hydroxide or other fused alkali metal compound, stable at the operating temperature. For example, I may use-a fused alkali metal hydroxide, such as caustic soda, various fused salts or mixture of fused salts, which have a sufficiently low melting point so as to be molten at the operating temperature. Various of the fused salts and mixtures suitable for heat treating metals may be used for this purpose- There is a large variety of such fused salt mixtures, well known to those experienced in treating metals and such need not be described in detail here. The fused salt baths suitable for I my process include the halides of the alkali and alkaline earth metals and mixtures thereof. Alkali metal hydroxides also may be mixed with the salts. Large concentrations of oxidizing salts such as nitrates are to be avoided, as these tend to attack the metals present.

Example I A bath of molten sodium-lead alloy containing 5.3% by weight of sodium was covered with a layer of fused sodium! hydroxide and heated to 620 C. Sheet steak-having a heavy coating of oxide scale was treated by immersion in the bath for two minutes. On-removal from the bath, the steel was cooled to room temperature in the air and then lightly brushed with a steel wire brush in a stream of hot water. The resulting surface did not rust when exposed to a humid, industrial atmosphere for several weeks. Some sheets were wire-brushed on one side only; In such, the unbrushed side rusted badly within a week, while the side which had been brushed showed no evidence of rust formation after standing several weeks.

My invention is not restricted to the methods described above and illustrated by the above ex-.

" ample, since it is obvious that various modifications thereof may be made without departing from the spirit and scope of this invention. For

example, the invention is not restricted to the descaling of ferrous metals such as iron or steel but is likewise applicable to the removable of oxide coatings from other metals such as copper. brass, nickel andthe like which can be operated on at the required temperatures. In place of pure sodium-lead alloy, lead alloys containing alkali metal with or without the addition of other ingredients such as antimony, tin, bismuth or the like may be used. In fact, any desired metallic ingredient may be added to the lead alloy bath as long as the bath remains fusible at the required temperature and the required amount of alkali metal remains in solution. Although it is preferable to use sodium as the alkalimetal in carrying out this invention, other alkali metals such as potassium, lithium or rubidium may be used in place of sodium.

An advantage of the present invention'over the methods heretofore used for removing oxides from metals is that the treating bath has no tendency to react with or to etch the underlying metal surface but reacts only with the oxide layer to be removed. The method has no adverse eflect upon the properties of the metal to be treated. In fact the bath may beused in some cases to improve the properties of the metal by annealing or other heat treating operations which may be simultaneously carried on by leaving the metal in the bath for a sumcient period of time. A further advantage is that ferrous metals may be treated according tom-y invention to form thereon an excellent rust-proof surface.

Iclaim: I, 7

1. The process for removing an oxide film from a ferrous metal surface which comprises bringing said surface into contact with a fused metal bath composed principally of a lead-sodium alloy containing 0.5 to 10% by weight of at a temperature of 600 to 800 C. and thereafter, before cooling,wiping any adhering fused metal or oxide from said ferrous metal surface.

2. Theprocess for removing an oxide him from 1 the surface of a ferrous metal which comlprises bringing said surface into contact with a'fused alloy composed principally of lead andcontaining at least 0.5% by weight of alkali metal, at' a temperature of not less than about 500 C. and

thereafter mechanically removing any adhering lead alloy from said surface without substantial abrasion of said surface.

3. The process for removing an oxide filmfrom the surface of-a ferrous metal which comprises bringing said surface into contact'with a fused alloy composed principally of lead and containing at least 0.5% by'weight of sodium,- at a temperature of not less than about 500 C. and thereafter removing any adhering lead alloy from said surface by subjecting said surface to brushing and washing with water.

4. The process for removing an oxide film fromthe surface of a ferrous metalsurface which comprises immersing said surface into a fused bath of lead-sodium alloy containing 0.5 to 10% by weight of sodium, at a temperature of 600 to 800 (7., while maintaining the surface of said bath out of direct contact with the air and thereafter removing any adhering lead alloy from said surface by washing said surface with hot waterand brushing it with a wire brush without substantial abrasion of said CEDRIC A. VINCENT-DAVISS. 

